Simulation of Coupled THMC Processes in Fractured Rocks Using a Dual-Media Approach

ABSTRACT: Simulation of coupled thermo-hydro-mechanical-chemical processes in fractured rocks is relevant to many areas of applied geoscience. Groundwater flow and reactive transport (HC processes) in hard rocks are primarily controlled by the network of rock fractures, and therefore best modelled in a discrete fracture network. By contrast, heat and stress (TM processes) are mediated via the rock mass, and thus best represented as a continuum. Here, we present an approach using dual, coincident meshes, each simulating the relevant processes in their ‘natural’ media. A sequential coupling between TM and HC processes is achieved by computing the effective stress on fractures and updating their hydraulic properties. The simulated processes are modelled in PFLOTRAN, using a linear elastic constitutive model for rock deformation. Several updates to the PFLOTRAN code were required to enable this, as verified against analytical cases and other numerical codes. We present an example application to a model of a nuclear waste repository in Finland, which simulates the evolution of thermal stress due to radiogenic heating from spent fuel canisters. The results illustrate how continuum-based TM processes influence near-field flows around the repository that can only be accurately captured in a DFN, thus demonstrating the advantages of a dual-media approach. 1. INTRODUCTION Simulation of coupled thermo-hydro-mechanical-chemical (THMC) processes in fractured rocks is relevant to many areas of applied geoscience and geo-engineering, including in nuclear waste disposal, enhanced geothermal systems, CO2 sequestration and mining applications. The ability to accurately predict the influence of coupled processes in the context of fractured media has potentially far-reaching economic and environmental consequences. In this study, we introduce a novel workflow for simulation of such processes, which employs a ‘dual-media’ representation of the modelled rock volume. In any numerical THMC model, a range of different couplings to geomechanical processes can feasibly be implemented. These include thermo-mechanical, hydro-mechanical and chemo-mechanical coupled processes. For the purposes of this study, the focus was primarily on the thermo-mechanical coupling between the rock's temperature and its stress state. The fundamental mechanism by which this is governed is the thermal expansion/contraction of the rock (and other modelled materials), as determined by the thermal expansion coefficient. Such expansion induces deformations, or ‘strains’, in the rock mass, which in turn bring about alterations in the stress field. A coupling to hydraulic processes (i.e. groundwater flow) can also be incorporated by computing the impact of thermal and mechanical strains on the rock's hydraulic properties – whether that be on the porosity field in the case of porous materials, or indeed on fracture apertures and transmissivities in the case of fractured rocks.